lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2011 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  19
  20 #ifndef _GL_VERIFY_H
  21 # define _GL_VERIFY_H
  22
  23
  24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per the
  25    C1X draft N1548 section 6.7.10.  This is supported by GCC 4.6.0 and
  26    later, in C mode, and its use here generates easier-to-read diagnostics
  27    when verify (R) fails.
  28
  29    Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per the
  30    C++0X draft N3242 section 7.(4).
  31    This will likely be supported by future GCC versions, in C++ mode.
  32
  33    Use this only with GCC.  If we were willing to slow 'configure'
  34    down we could also use it with other compilers, but since this
  35    affects only the quality of diagnostics, why bother?  */
  36 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus
  37 #  define _GL_HAVE__STATIC_ASSERT 1
  38 # endif
  39 /* The condition (99 < __GNUC__) is temporary, until we know about the
  40    first G++ release that supports static_assert.  */
  41 # if (99 < __GNUC__) && defined __cplusplus
  42 #  define _GL_HAVE_STATIC_ASSERT 1
  43 # endif
  44
  45 /* Each of these macros verifies that its argument R is nonzero.  To
  46    be portable, R should be an integer constant expression.  Unlike
  47    assert (R), there is no run-time overhead.
  48
  49    If _Static_assert works, verify (R) uses it directly.  Similarly,
  50    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  51    that is an operand of sizeof.
  52
  53    The code below uses several ideas for C++ compilers, and for C
  54    compilers that do not support _Static_assert:
  55
  56    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  57      integral or boolean or floating-point type, this yields an
  58      expression of integral type, whose value is later verified to be
  59      constant and nonnegative.
  60
  61    * Next this expression W is wrapped in a type
  62      struct _gl_verify_type {
  63        unsigned int _gl_verify_error_if_negative: W;
  64      }.
  65      If W is negative, this yields a compile-time error.  No compiler can
  66      deal with a bit-field of negative size.
  67
  68      One might think that an array size check would have the same
  69      effect, that is, that the type struct { unsigned int dummy[W]; }
  70      would work as well.  However, inside a function, some compilers
  71      (such as C++ compilers and GNU C) allow local parameters and
  72      variables inside array size expressions.  With these compilers,
  73      an array size check would not properly diagnose this misuse of
  74      the verify macro:
  75
  76        void function (int n) { verify (n < 0); }
  77
  78    * For the verify macro, the struct _gl_verify_type will need to
  79      somehow be embedded into a declaration.  To be portable, this
  80      declaration must declare an object, a constant, a function, or a
  81      typedef name.  If the declared entity uses the type directly,
  82      such as in
  83
  84        struct dummy {...};
  85        typedef struct {...} dummy;
  86        extern struct {...} *dummy;
  87        extern void dummy (struct {...} *);
  88        extern struct {...} *dummy (void);
  89
  90      two uses of the verify macro would yield colliding declarations
  91      if the entity names are not disambiguated.  A workaround is to
  92      attach the current line number to the entity name:
  93
  94        #define _GL_CONCAT0(x, y) x##y
  95        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
  96        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
  97
  98      But this has the problem that two invocations of verify from
  99      within the same macro would collide, since the __LINE__ value
 100      would be the same for both invocations.  (The GCC __COUNTER__
 101      macro solves this problem, but is not portable.)
 102
 103      A solution is to use the sizeof operator.  It yields a number,
 104      getting rid of the identity of the type.  Declarations like
 105
 106        extern int dummy [sizeof (struct {...})];
 107        extern void dummy (int [sizeof (struct {...})]);
 108        extern int (*dummy (void)) [sizeof (struct {...})];
 109
 110      can be repeated.
 111
 112    * Should the implementation use a named struct or an unnamed struct?
 113      Which of the following alternatives can be used?
 114
 115        extern int dummy [sizeof (struct {...})];
 116        extern int dummy [sizeof (struct _gl_verify_type {...})];
 117        extern void dummy (int [sizeof (struct {...})]);
 118        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 119        extern int (*dummy (void)) [sizeof (struct {...})];
 120        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 121
 122      In the second and sixth case, the struct type is exported to the
 123      outer scope; two such declarations therefore collide.  GCC warns
 124      about the first, third, and fourth cases.  So the only remaining
 125      possibility is the fifth case:
 126
 127        extern int (*dummy (void)) [sizeof (struct {...})];
 128
 129    * GCC warns about duplicate declarations of the dummy function if
 130      -Wredundant_decls is used.  GCC 4.3 and later have a builtin
 131      __COUNTER__ macro that can let us generate unique identifiers for
 132      each dummy function, to suppress this warning.
 133
 134    * This implementation exploits the fact that older versions of GCC,
 135      which do not support _Static_assert, also do not warn about the
 136      last declaration mentioned above.
 137
 138    * In C++, any struct definition inside sizeof is invalid.
 139      Use a template type to work around the problem.  */
 140
 141 /* Concatenate two preprocessor tokens.  */
 142 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 143 # define _GL_CONCAT0(x, y) x##y
 144
 145 /* _GL_COUNTER is an integer, preferably one that changes each time we
 146    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 147    otherwise.  __LINE__ isn't perfect, but it's better than a
 148    constant.  */
 149 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 150 #  define _GL_COUNTER __COUNTER__
 151 # else
 152 #  define _GL_COUNTER __LINE__
 153 # endif
 154
 155 /* Generate a symbol with the given prefix, making it unique if
 156    possible.  */
 157 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 158
 159 /* Verify requirement R at compile-time, as an integer constant expression
 160    that returns 1.  If R is false, fail at compile-time, preferably
 161    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 162
 163 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 164     (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 165
 166 # ifdef __cplusplus
 167 template <int w>
 168   struct _gl_verify_type {
 169     unsigned int _gl_verify_error_if_negative: w;
 170   };
 171 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 172     _gl_verify_type<(R) ? 1 : -1>
 173 # elif defined _GL_HAVE__STATIC_ASSERT
 174 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 175      struct {                                   \
 176        _Static_assert (R, DIAGNOSTIC);          \
 177        int _gl_dummy;                          \
 178      }
 179 # else
 180 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 181      struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 182 # endif
 183
 184 /* Verify requirement R at compile-time, as a declaration without a
 185    trailing ';'.  If R is false, fail at compile-time, preferably
 186    with a diagnostic that includes the string-literal DIAGNOSTIC.
 187
 188    Unfortunately, unlike C1X, this implementation must appear as an
 189    ordinary declaration, and cannot appear inside struct { ... }.  */
 190
 191 # ifdef _GL_HAVE__STATIC_ASSERT
 192 #  define _GL_VERIFY _Static_assert
 193 # else
 194 #  define _GL_VERIFY(R, DIAGNOSTIC)                                    \
 195      extern int (*_GL_GENSYM (_gl_verify_function) (void))             \
 196        [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 197 # endif
 198
 199 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 200 # ifdef _GL_STATIC_ASSERT_H
 201 #  if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
 202 #   define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
 203 #  endif
 204 #  if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
 205 #   define static_assert _Static_assert /* Draft C1X requires this #define.  */
 206 #  endif
 207 # else
 208
 209 /* Each of these macros verifies that its argument R is nonzero.  To
 210    be portable, R should be an integer constant expression.  Unlike
 211    assert (R), there is no run-time overhead.
 212
 213    There are two macros, since no single macro can be used in all
 214    contexts in C.  verify_true (R) is for scalar contexts, including
 215    integer constant expression contexts.  verify (R) is for declaration
 216    contexts, e.g., the top level.  */
 217
 218 /* Verify requirement R at compile-time, as an integer constant expression.
 219    Return 1.  */
 220
 221 #  define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
 222
 223 /* Verify requirement R at compile-time, as a declaration without a
 224    trailing ';'.  */
 225
 226 #  define verify(R) _GL_VERIFY (R, "verify (" #R ")")
 227
 228 # endif
 229
 230 #endif